Members of the SRC family of non-receptor protein tyrosine kinases (SFKs) play crucial roles in the regulation of cellular growth, differentiation, and morphology. Studies in vertebrates have shown that inappropriate activation of SFKs can lead to oncogenic transformation of cells, while the absence of SFKs has been associated with defects in signaling by the B and T cell antigen receptors and with defects in bone resorption (osteopetrosis). Among the important cellular consequences of altered SFK function are defects in the cellular actin cytoskeleton. The aim of the proposed research is to investigate how SFKs participate in cytoskeletal regulation. We are addressing this issue by studying the action of a particular SFK, the product of the Drosophila Src64 gene. We have chosen to study Src64 because of the ability to perform genetic studies that would be either difficult or impossible to conduct in vertebrate organisms. Our goal is to combine these genetic studies with extensive biochemical analysis in order to understand Src64 function. In our previous work, we identified mutations that inactivated the Src64 gene. We showed that the lack of Src64 function has a surprisingly specific phenotype: Src64 mutant animals are fully viable, but the females are partially sterile. Our analysis of this phenotype revealed that it is associated with defects in the morphogenesis of specialized actin cytoskeletal structures, called ring canals, that form at the cytoplasmic bridges connecting the developing oocyte and its adjacent cluster of nurse cells. We then genetically identified the Tec29 protein as a major target of SRC64 action. ThC29 is a member of a family of tyrosine kinases that includes vertebrate proteins such as Bruton's tyrosine kinase, whose absence in humans leads to X-linked agammaglobulinemia. We showed that TEC29 is essential for normal ring canal morphogenesis and is recruited to the growing ring canals in response to SRC64 activity. The goals of the proposed research are to further characterize the role of SRC64 and TBC29 in actin cytoskeletal regulation by: 1) investigating the role of SRC64 and ThC29 using domain specific mutagenesis, 2) investigating the biochemical basis for SRC64 regulation of TEC29, 3) identifying and characterizing substrates of TEC29, 4) cloning and analyzing E(Src64)2B, another potential target of SRC64, and 5) conducting further genetic screens to identify additional components of the SRC64/TEC29 pathway.